Sunday, August 7, 2011

Apsical Precession & the Length of Summer

Apsidal precession causes small changes in the length of Summer as the images below show. The plot gives the number of days of from the Vernal Equinox to the Autumnal Equinox as a function of the angle of perhelion from the Winter Solstice. The result is a peak-to-peak variation of about 8 days.

This is only a 2% change but could have a greater affect at higher latitudes where thermal cycling is the greatest.

If the perihelion is at midsummer one might expect a slightly greater intensity of solar radiation since the Earth would be closer to the Sun at this time of year. Winters in the Northern Hemisphere may on average be slightly milder at this time than at other times in the 21,000 year apsidal precession cycle. The apsidal precession is barely noticeable since it is masked by the shifts caused by a leap day every four years. It takes on average about 57.5 years for the date of perihelion to move one day backwards in the year so it may be just noticeable within the span of a lifetime.

No comments: